CONTINUOUS SINGLE-DIP PROCESS FOR GALVANIZATION OF STEEL LONG PRODUCTS INTO Zn-Al-Mg ALLOYS

ABSTRACT

By first fluxing a steel long product with novel specific flux compositions, it is possible to continuously produce, more uniform, smoother and void-free galvanized coatings on such steel long products in a single hot dip galvanization step making use of zinc-aluminum alloys or zinc-aluminum-magnesium alloys with less than 95 wt. % zinc. This is achieved by providing specific amounts of lead chloride and tin chloride in a flux composition comprising (a) more than 40 and less than 70 weight % zinc chloride, (b) from 10 to 30 weight % ammonium chloride, (c) more than 6 and less than 30 weight % of a set of at least two alkali or alkaline earth metal chlorides.

This application claims the benefit of British Patent Application No.1219210.0 filed Oct. 25, 2012, the disclosure of which is incorporatedby reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of steel metallurgy, inparticular galvanization, more specifically the galvanization or zinccoating of steel long products such as, but not limited to, wires,reinforcing bars (sometimes referred in the art as “rebars”), rods,rails, bars, structural shapes, tubes and the like. In particular thepresent invention relates to a continuous process for the galvanizationof ferrous filamentous materials such as steel wires. The presentinvention also relates to galvanized steel long products (e.g. wires,rods, rails, bars and tubes) being galvanized continuously with the aidof novel fluxing compositions.

BACKGROUND OF THE INVENTION

Within the framework of steel metallurgy, long products are made fromsemi-finished products such as blooms (cross-section usually from 125 to400 mm²) and billets (cross-section usually from 50 to 125 mm²) bycasting with a continuous caster or rolling at a blooming mill.

As used herein throughout this application, long products are productswith one dimension (length) being at least 10 times higher than the twoother dimensions (as opposed to flat products) and include bars, rods,wires (coiled or not, for making e.g. bolts and fences), structuralshapes and sections, rails, pipes, and tubes, e.g. for use in civilconstruction, mechanical engineering, energy, transport (railway,tramway), household and furniture. Bars are long products with square,rectangular, flat, round, or polygonal cross sections. Rounds can reacha diameter of about 250 mm. They are sometimes cold-drawn or even groundto very precise dimensions for use in machine parts. A special group ofrounds are the reinforcing bars. Produced in diameters from about 10 to75 mm, they provide tensile strength to concrete sections subjected to abending load. They normally have hot-rolled protrusions on their surfaceto improve bonding with concrete.

Hot-rolled wire rods are produced in diameters between about 5 and 15 mmand may be shipped in coils. Rods may be cold-drawn into wires which maybe afterwards covered by a coating for corrosion protection. The use ofwire is extremely wide, ranging from cords for belted tires to cablesfor suspension bridges.

The most common structural shapes are wide flange I-beams, H-beams,L-beams, and T-beams. Such shapes are standardized, and may includerailroad rails and special rails, e.g. for cranes and heavy transfercars or for use in mines and construction.

Tubular steel long products may be broadly grouped into welded andseamless products. Longitudinally welded tubes are normally produced upto about 500 mm in diameter and/or about 10 mm in wall thickness. Pipesproduced from heavy plates are also longitudinally welded after beingformed and can be from about 0.5 m to 2 m in diameter, with a wallthickness up to about 180 mm. Seamless tubes are usually subjected tomore demanding service; and may be rolled in diameters ranging from 120to 400 mm and/or in a wall thickness up to about 15 mm, although specialrolling mills can often increase their diameter to 650 mm. Smallerdiameter tubes, both welded and seamless, can be produced by reductionmills or cold-drawing benches. Tubes are frequently machined on bothends for various coupling systems and coated with organic material.

The importance of providing protection against corrosion for ferrous(e.g. iron or steel) long products used under harsh environmentalconditions, e.g. outdoors, is well known. Coating a ferrous material(mainly steel) with zinc is a very effective and economical means foraccomplishing this goal. Zinc coatings are commonly applied by dippingor passing the steel long product to be coated through a molten bath ofthe metal. This operation is termed “galvanizing”, “hot galvanizing” or“hot-dip galvanizing” (HDG) to distinguish it from zinc electroplatingprocesses. In this process, a solidified layer of zinc is formed on theproduct surface and the zinc coating layer formed as a result isstrongly adhered to the surface of the article by an iron/zincintermetallic alloy which forms during the galvanizing process. It iswell known that oxides and other foreign materials (“soil”) on thesurface of the steel article interfere with the chemistry of thegalvanizing process and prevent formation of a uniform, continuous,void-free coating. Accordingly, various techniques and combinations oftechniques have been adopted in industry to reduce, eliminate, or atleast accommodate, oxides and soil as much as possible.

Improvement in the properties of galvanized steel products can beachieved by alloying zinc with aluminum, and optionally magnesium. Forinstance addition of 5 wt. % aluminum produces an alloy with a lowermelting temperature (eutectic point at 381° C.) which exhibits improveddrainage properties relative to pure zinc. Moreover, galvanized coatingsproduced from this zinc-aluminum alloy (known as Galfan, subject tostandard specifications such as ASTM B 750-99, ASTM A 856-98) havegreater corrosion resistance, improved formability and betterpaintability in comparison to a conventional galvanized coating, i.e.formed from pure zinc. Galfan coatings advantageously combine thepassive corrosion inhibition of aluminum oxidation with the active andpassive effects of zinc. Galfan-coated wires may be drawn (subject tostandard specification ASTM A 764) into spring wires, strands (standardspecification ASTM A 855), chain link fences (standard specificationsASTM A 817-94 and ASTM A 824-95), gabions (standard specification ASTM A974-97), and steel-reinforced aluminum conductors (standardspecifications ASTM B 232-99 and ASTM B 401-99). Further advantages ofGalfan coated wires, vis-à-vis conventional galvanized wire, have beenevidenced for steel springs, including consistency of spring length(associated with a decreased frictional interaction with coiling tools),and good adherence of the Galfan coating to organic coatings. However,zinc-aluminum galvanizing is known to be particularly sensitive tosurface cleanliness, so that various difficulties, such as insufficientsteel surface wetting and the like, are often encountered whenzinc-aluminum alloys are used in galvanizing.

Many techniques and combinations thereof have been adopted in industryto reduce, eliminate, or at least accommodate, oxides and soil as muchas possible. In essentially all these processes, organic soil, that is,oil, grease, rust preventive compounds, is first removed by contactingthe surface to be coated with an alkaline aqueous wash (alkalinecleaning). This may be accompanied by additional techniques such asbrush scrubbing, ultrasound treatment and/or electro-cleaning, ifdesired. Then follows rinsing with water, contacting the surface with anacidic aqueous wash for removing iron fines and oxides (pickling), andfinally rinsing with water again. All these cleaning-pickling-rinsingprocedures are common for most galvanizing techniques and areindustrially carried out more or less accurately.

Another pre-treatment method used for high strength steels, steels withhigh carbon contents, cast iron and cast steels is a mechanical cleaningmethod called blasting. In this method, rust and dirt are removed fromthe steel or iron surface by projecting small shots and grits onto thissurface. Depending on the shape, size and thickness of the parts to betreated, different blasting machines are used such as a tumble blastingmachine for bolts, a tunnel blasting machine for automotive parts, etc.

There are two main galvanizing techniques used on cleaned metal (e.g.iron or steel) parts: (1) the fluxing method, and (2) the annealingfurnace method.

The first galvanizing technique, i.e. the fluxing method, may itself bedivided into two categories, the dry fluxing method and the wet fluxingmethod.

The dry fluxing method, which may be used in combination with one ormore of the above cleaning, pickling, rinsing or blasting procedures,creates a salt layer on the ferrous metal surface by dipping the metalpart into an aqueous bath containing chloride salts, called a“pre-flux”. Afterwards, this layer is dried prior to the galvanizingoperation, thus protecting the steel surface from re-oxidation until itsentrance in a molten zinc bath. Such pre-fluxes normally compriseaqueous zinc chloride and optionally contain ammonium chloride, thepresence of which has been found to improve wettability of the articlesurface by molten zinc and thereby promote formation of a uniform,continuous, void-free coating.

The concept of wet fluxing is to cover the galvanizing bath with a topflux also typically comprising zinc chloride, and usually ammoniumchloride, but in this case these salts are molten and are floating onthe top of the galvanizing bath. The purpose of a top flux, like apre-flux, is to supply zinc chloride and preferably ammonium chloride tothe system to aid wettability during galvanizing. In this case, allsurface oxides and soil which are left after cleaning-pickling-rinsingare removed when the steel part passes through the top flux layer and isdipped into the galvanizing kettle. Wet fluxing has severaldisadvantages such as, consuming much more zinc than dry fluxing,producing much more fumes, etc. Therefore, the majority of galvanizingplants today have switched their process to the dry fluxing method.

Below is a summary of the annealing furnace method. In continuousprocesses using zinc or zinc-aluminum or zinc-aluminum-magnesium alloysas the galvanizing medium, annealing is done under a reducing atmospheresuch as a mixture of nitrogen and hydrogen gas. This not only eliminatesre-oxidation of previously cleaned, pickled and rinsed surfaces but,also actually removes any residual surface oxides and soil that mightstill be present. The majority of steel coils are today galvanizedaccording to this technology. A very important requirement is that thecoil is leaving the annealing furnace by continuously going directlyinto the molten zinc without any contact with air. However thisrequirement makes it extremely difficult to use this technology forshaped parts, or for steel wire since wires break too often and theannealing furnace method does not allow discontinuity.

Another technique used for producing zinc-aluminum galvanized coatingscomprises electro-coating the steel articles with a thin (i.e. 0.5-0.7μm) layer of zinc (hereafter “pre-layer”), drying in a furnace with anair atmosphere and then dipping the pre-coated article into thegalvanizing kettle. This is widely used for hot-dip coating of steeltubing in continuous lines and to a lesser extent for the production ofsteel strip. Although this does not require processing under reducingatmospheres, it is disadvantageous because an additional metal-coatingstep required.

Galvanizing is practiced either in batch operation or continuously.Continuous operation is suitably practiced on steel long products suchas wires, tubes, rods and rails. In continuous operation, transfer ofthe articles between successive treatments steps is very fast and donecontinuously and automatically, with operating personnel being presentto monitor operations and fix problems if they occur. Production volumesin continuous operations are high. In a continuous galvanizing lineinvolving use of an aqueous pre-flux followed by drying in a furnace,the time elapsing between removal of the article from the pre-flux tankand dipping into the galvanizing bath is usually about 10 to 60 seconds.

There is a need to combine good formability with enhanced corrosionprotection of the ferrous metal article. However, before a zinc-basedalloy coating with high amounts of aluminum (and optionally magnesium)can be introduced into the general galvanizing industry, the followingdifficulties have to be overcome:

-   -   zinc alloys with high aluminum contents can hardly be produced        using the standard zinc-ammonium chloride flux. Fluxes with        metallic Cu or Bi deposits have been proposed earlier, but the        possibility of copper or bismuth leaching into the zinc bath is        not attractive. Thus, better fluxes are needed.    -   high-aluminum content alloys tend to form outbursts of zinc-iron        intermetallic alloy which are detrimental at a later stage in        the galvanization. This phenomenon leads to very thick,        uncontrolled and rough coatings. Control of outbursts is        absolutely essential.    -   wettability issues were previously reported in Zn—Al alloys with        high-aluminum content, possibly due to a higher surface tension        than pure zinc. Hence bare spots due to a poor wetting of steel        are easily formed, and hence a need to lower the surface tension        of the melt.    -   a poor control of coating thickness was reported. in Zn—Al        alloys with high-aluminum content, possibly depending upon        parameters such as temperature, flux composition, dipping time,        steel quality, etc.

Thus a lot of technical problems remain to be solved in the steelgalvanizing industry. Furthermore there are also problems which arespecific to the galvanization of steel long products. Molten Galfanalloy is not compatible with most flux systems conventionally used ingalvanizing. This limitation has led to wide usage of “double dipping”processes wherein the Galfan hot dip follows a conventional hot dip. Forthe proper galvanization of steel wires with a zinc-aluminum orzinc-aluminum-magnesium alloy, it is thus usually necessary to rely onthe so-called double-dip technology, i.e. first dipping the steel longproduct into a zinc bath, and then dipping the zinc-coated steel wireinto a second zinc-aluminum or zinc-aluminum-magnesium alloy bath. Inthis double dip processing the properly annealed, cleaned and fluxedsteel acquires a galvanized coating in the first bath This coating willgenerally include a series of iron-zinc intermetallic compounds at theiron-zinc interface, together with an overlay that is nearly pure zinc.The series of iron-zinc intermetallic compounds can be a source ofcoating brittleness. When the galvanized steel long product enters thesecond bath containing molten Galfan, the bath temperature willgenerally be high enough to melt or dissolve the essentially zincgalvanized overlay and transform the iron-zinc intermetallic layer intoan aluminum-iron-zinc intermetallic. Upon emergence from the Galfan batha layer of essentially Galfan alloy solidifies on top of the transformedaluminum-iron-zinc intermetallic layer. Aluminum that enters into thealuminum-iron-zinc intermetallic layer inherently lowers the aluminumconcentration in the second bath. Thus double dip processing requiresprecise monitoring and management of the aluminum concentration.

Such a double dip processing appears for instance in EP 1.158.069disclosing a plated steel wire wherein the average composition of theplating alloy used in the second stage contains 4-20 wt. % Al, 0.8-5 wt.% Mg and the balance Zn, and wherein an Fe—Zn alloy layer of no greaterthan 20 μm thickness is present at the plating-base metal interface.Such wire coating double dip procedure suffers from many technical andeconomical disadvantages as follows:

-   -   the need to invest into two separate zinc-based baths,    -   a higher energetic consumption than with a single bath procedure        since wires need to be heated twice, and be quickly cooled down        in between the two process stages,    -   the difficulty and extra cost to maintain the aluminum content        (and optionally the magnesium content) constant in the second        zinc-based bath, as reported for instance by Frank Goodwin and        Roger Wright in The process metallurgy of zinc-coated steel wire        and Galfan bath management jointly published by International        Lead Zinc Research Organization Inc (North Carolina, U.S.A) and        Rensselaer Polytechnic Institute (Troy, N.Y., U.S.A).    -   a higher residence time of wires at high temperature than with a        single bath procedure and consequently a higher loss of        mechanical resistance (tensile strength).

WO 03/057940 discloses a process for the preparation of a steel surfacefor hot-dip galvanizing in an aluminum-rich zinc-based (e.g. Galfan)molten bath, comprising the steps consisting of electrocleaning,ultrasonic cleaning or mechanical brush cleaning the surface, picklingthe surface, and applying a protective layer to the surface by immersionin a flux solution, characterized in that cleaning is performed so as toobtain less than 0.6 μg/cm² residual dirt, and the flux solutioncomprises a soluble bismuth compound. Although a bismuth-containing fluxcomposition may provide good Galfan coating at speeds which arecompatible with a continuous production line for the galvanization ofwires, it also suffers significant disadvantages such as veryrestrictive conditions of the previous cleaning or pickling steps. WO03/057940 also teaches that coating quality significantly decreases whenthe aluminum content in the zinc-based galvanization bath increases, andfurther experiments have shown that this technology becomes hardlypracticable when the aluminum content in the zinc-based galvanizationbath exceeds 5 wt. % and/or when the zinc-based galvanization bathfurther includes magnesium.

It is known in the art that the addition of magnesium to analuminum-rich zinc-based galvanization bath enhances the corrosionresistance, especially in a saline environment, and that this beneficialeffect is greater when the magnesium concentration increases. However itis also known in the art that magnesium addition in a zinc alloy bathmay decrease the cracking resistance of the coating being formed. Themain factor for this phenomenon appears to be the formation of anintermetallic compound MgZn₂, the cracking resistance of which is lowunder the influence of mechanical stress. Furthermore magnesium additionin a zinc alloy bath leads to the formation of a relatively roughcoating microstructure. Stress repartition within the coating beingformed is consequently less homogeneous, and more important stress mayappear at the interface of the different metallic phases constitutingthe coating. Thus, not only magnesium addition improves corrosionresistance at the expense of some manufacturing problems and of thecoating quality, but also it tends to increase the formation of soil ordross which float at the surface of the zinc bath, as evidenced forinstance in FIG. 1 of European patent No. 1.158.069.

WO 2011/009999 solves the above problems of magnesium addition byproviding a coated long product, in particular a steel wire, by dippingit into a zinc alloy bath including 4-8 wt. % aluminum and 0.2-0.7 wt. %magnesium and, upon exit from said bath, cooling the coated product,wherein said cooling is controlled to impart to said metal coating ahomogeneous microstructure having more than 25% by volume of a betaphase portion being responsible for a good ductility of the coatinglayer.

WO 02/42512 describes a flux for hot dip galvanization comprising 60-80wt. % zinc chloride; 7-20 wt. % ammonium chloride; 2-20 wt. % of atleast one alkali or alkaline earth metal salt; 0.1-5 wt. % of a leastone of NiCl₂, CoCl₂ and MnCl₂; and 0.1-1.5 wt. % of at least one ofPbCl₂, SnCl₂, SbCl₃ and BiCl₃. Preferably this flux comprises 6 wt. %NaCl and 2 wt. % KCl. Examples 1-3 teach flux compositions comprising0.7-1 wt. % lead chloride.

WO 2007/146161 describes a method of galvanizing with a moltenzinc-alloy comprising the steps of (1) immersing a ferrous material tobe coated in a flux bath in an independent vessel thereby creating aflux coated ferrous material, and (2) thereafter immersing the fluxcoated ferrous material in a molten zinc-aluminum alloy bath in aseparate vessel to be coated with a zinc-aluminum alloy layer, whereinthe molten zinc-aluminum alloy comprises 10-40 wt. % aluminum, at least0.2 wt. % silicon, and the balance being zinc and optionally comprisingone or more additional elements selected from the group consisting ofmagnesium and a rare earth element. In step (1), the flux bath maycomprise from 10-40 wt. % zinc chloride, 1-15 wt. % ammonium chloride,1-15 wt. % of an alkali metal chloride, a surfactant and an acidiccomponent such that the flux has a final pH of 1.5 or less. In anotherembodiment of step (1), the flux bath may be as defined in WO 02/42512.

JP 2001/049414 describes producing a hot-dip Zn—Mg—Al base alloy coatedsteel sheet excellent in corrosion resistance by hot-dipping in a fluxcontaining 61-80 wt. % zinc chloride, 5-20 wt. % ammonium chloride, 5-15wt. % of one or more chloride, fluoride or silicafluoride of alkali oran alkaline earth metal, and 0.01-5 wt. % of one or more chlorides ofSn, Pb, In, TI, Sb or Bi. More specifically, table 1 of JP 2001/049414discloses various flux compositions with a KCl/NaCl weight ratio rangingfrom 0.38 to 0.60 which, when applied to a steel sheet in a molten alloybath comprising 0.05-7 wt. % Mg, 0.01-20 wt. % Al and the balance beingzinc, provide a good plating ability, no pin hole, no dross, and flat.By contrast, table 1 of JP 2001/049414 discloses a flux composition witha KCl/NaCl weight ratio of 1.0 which, when applied to a steel sheet in amolten alloy bath comprising 1 wt. % Mg, 5 wt. % Al and the balancebeing zinc, provides a poor plating ability, pin hole defect, somedross, and poorly flat.

Although the methods described in the above documents have brought someimprovements over the previous state of the art, they have still notresolved most of the technical problems outlined hereinbefore,especially the numerous problems associated with the double dippingprocessing, with respect to the galvanization of steel long productssuch as, but not limited to, wires, rods, bars, rails, tubes, structuralshapes and the like.

Consequently there is still a need in the art for improving continuousprocessing conditions vis-à-vis the current double dip technique ofgalvanizing wires, as well as fluxing compositions used therefore.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an economically andtechnically improved method or process for the galvanization of steellong products such as, but not limited to, wires, bars, rods, rails,tubes and the like. It has been surprisingly found that, by firstfluxing a steel long product with novel specific flux compositions, itis possible to continuously produce, more uniform, smoother andvoid-free galvanized coatings on such steel long products in a singlehot dip galvanization step making use of zinc alloys, in particularzinc-aluminum alloys and zinc-aluminum-magnesium alloys of any suitablecomposition. More specifically it has surprisingly been found that thisobject can be achieved by providing both lead chloride and tin chloridein specific amounts in a flux composition comprising (a) more than 40and less than 70 wt. % zinc chloride, (b) from 10-30 wt. % ammoniumchloride, and (c) more than 6 and less than 30 wt. % of a set of atleast two alkali metal chlorides. The accurate selection of such fluxingcompositions provide the unexpected advantage of avoiding, during thegalvanization step, the need for a double dip processing sequentiallyusing two different zinc baths, and consequently avoiding the cumbersomealuminum (and optionally magnesium) concentration management required bythe current technique. The hereinabove stated technical and economicalproblems associated with double dip processing, or with thebismuth-containing fluxing compositions of WO 03/057940, are thus solvedby the continuous process defined in claim 1 and, more specificembodiments defined in claims 2-16, and the galvanized steel longproduct defined in claims 17-19.

DETAILED DESCRIPTION OF THE INVENTION

As defined in claim 1, the essential feature of this invention is therecognition that huge improvements in the galvanization of steel longproducts can be achieved when, after one or more treatment stepsselected from alkaline cleaning, rinsing, pickling and drying, thefurther fluxing step starts from a flux composition comprising both leadchloride and tin chloride in specified respective amounts and with aproviso that their combined amounts exceed a certain threshold beingabove what was previously known from the literature. This feature isassociated with specific amounts of the other flux components.

Definitions

The term “hot dip galvanization” is meant to designate the corrosiontreatment of a metal article such as, but not limited to, a steelproduct by dipping into a molten bath of an aluminum-rich zinc-basedalloy, in continuous operation, for a sufficient period of time tocreate an effective protective layer at the surface of said longproduct.

The term “long product” is as meant in the Background of the Invention,both generally and including the specific embodiments listed therein.

In the following the different percentages relate to the proportion byweight of each component with respect to the total weight (100%) of theflux composition. This implies that not all maximum or not all minimumpercentages can be present at the same time, in order for their sum tomatch to 100% by weight.

As defined in claim 1 the flux composition used in the fluxing step ofthis invention comprises, as an essential feature, from 0.1 to 2 weight% lead chloride and from 2 to 15 weight % tin chloride, with the provisothat the combined amounts of lead chloride and tin chloride represent atleast 2.5 weight % of said composition. Various embodiments of the fluxcomposition are further presented in more details.

In one embodiment, the proportion of lead chloride is at least 0.4 wt. %or at least 0.7 wt. %. In another embodiment, the proportion of leadchloride in the flux composition is at most 1.5 wt. % or at most 1.2 wt.%. In another embodiment, the proportion of lead chloride in the fluxcomposition is from 0.8 to 1.1 wt. %.

In one embodiment the proportion of tin chloride in the flux compositionis at least 3.5 wt % or at least 7 wt. %. In another embodiment, theproportion of tin chloride in the flux composition is at most 14 wt. %.

In one embodiment, the combined respective amounts of lead chloride andtin chloride represent at most 14 wt. % of the flux composition. Inanother embodiment, the flux composition may further comprise othersalts of lead and/or tin, such as the fluoride, or other chemicals thatare inevitable impurities present in commercial sources of lead chlorideand/or tin chloride.

In one aspect of this invention, the specified respective amounts oflead chloride and tin chloride are combined with specified proportionsof other chlorides that make it possible to produce continuous, moreuniform, smoother and void-free coatings on steel long products by asingle hot dip galvanization continuous process with a moltenaluminum-rich zinc-based alloy.

For instance, the specified respective amounts of lead chloride and tinchloride are combined with more than 40 and less than 70 wt. % zincchloride. In one embodiment, the proportion of zinc chloride in the fluxcomposition is at least 45 wt. % or at least 50 wt. %. In anotherembodiment, the proportion of zinc chloride in the flux composition isat most 65 wt. % or at most 62 wt. %. These selected proportions ofZnCl₂ are capable, in combination with the specified respective amountsof lead chloride and tin chloride in the flux composition, to ensure agood coating of the metal article to be galvanized and to effectivelyprevent oxidation of the metal article during subsequent process stepssuch as drying, i.e. prior to galvanization itself.

In one aspect of this invention, the specified respective amounts oflead chloride and tin chloride are combined with 10-30 wt. % ammoniumchloride. In one embodiment, the proportion of NH₄Cl in the fluxcomposition is at least 13 wt. % or at least 17 wt. %. In anotherembodiment, the proportion of ammonium chloride in the flux compositionis at most 26 wt. % or at most 22 wt. %. The optimum proportion of NH₄Clmay be determined by the skilled person, without extensiveexperimentation and depending upon parameters such as the metal to begalvanized and the weight proportions of the metal chlorides in the fluxcomposition, by simply using the experimental evidence shown in thefollowing examples, to achieve a sufficient etching effect during hotdipping to remove residual rust or poorly pickled spots, while howeveravoiding the formation of black spots, i.e. uncoated areas of the metalarticle. In some circumstances it may be useful to substitute a minorpart (e.g. less than ⅓ by weight) of NH₄Cl with one or more alkylquaternary ammonium salt(s) wherein at least one alkyl group has from 8to 18 carbon atoms such as described in EP 0488.423, for instance analkyl-trimethylammonium chloride (e.g. trimethyllauryl-ammoniumchloride) or a dialkyldimethylammonium chloride.

In one aspect of this invention, the specified respective amounts oflead chloride and tin chloride are combined with suitable amounts of oneor more, preferably several, alkali or alkaline earth metal halides.These halides are preferably or predominantly chlorides (still fluoridesmay be useful as well), and the alkali or alkaline earth metals areadvantageously selected (sorted in decreasing order of preference ineach metal class) from the group consisting of Na, K, Li, Cs, Mg, Ca, Srand Ba. The flux composition shall advantageously comprise a mixture ofthese alkali or alkaline earth metal halides. Such mixtures tend toincrease the average chemical affinity of the molten mixture towardschlorine and to provide a synergistic effect allowing to better and moreaccurately control the melting point and the viscosity of the moltensalts and hence their wettability. In one embodiment, the mixture ofalkali or alkaline earth metal halides is a set of at least two alkalimetal chlorides and represents from 10 to 30 wt. % of the fluxcomposition. In another embodiment, the set of at least two alkali metalchlorides includes sodium chloride and potassium chloride as majorcomponents. In another embodiment, the set of at least two alkali metalchlorides (e.g. NaCl and KCl as major components) represents at least 12wt. % or at least 15 wt. % of the flux composition. In anotherembodiment, the set of at least two alkali metal chlorides (e.g.including sodium chloride and potassium chloride as major components)represents at most 25 wt. %, or at most 21 wt. %, of the fluxcomposition. In a specific embodiment, the proportion of the at leasttwo alkali metal chlorides (e.g. including sodium chloride and potassiumchloride as major components) in the flux composition is from 20 wt. %to 25 wt. %. NaBr, KBr, MgCl₂ and/or CaCl₂ may be present as minorcomponents in each of the above stated embodiments.

In order to achieve the best possible advantages, the ratio betweenthese alkali or alkaline earth metal halides in their mixtures is notwithout importance. As is known from the prior art the mixture of alkalior alkaline earth metal halides may be a set of at least two alkalimetal chlorides including sodium chloride and potassium chloride in aKCl/NaCl weight ratio from 0.2 to 1.0. In one embodiment, the KCl/NaClweight ratio may be from 0.25 to 0.6. In one embodiment, the KCl/NaClweight ratio may be from 1.0 to 2.0. It has also been surprisingly foundthat flux compositions wherein the mixture of alkali or alkaline earthmetal halides is a set of at least two alkali metal chlorides includingsodium chloride and potassium chloride in a KCl/NaCl weight ratio from2.0 to 8.0 exhibit outstanding properties. In anyone embodiment, theKCl/NaCl weight ratio may be from 3.5 to 5.0, or from 3.0 to 6.0.

In one aspect of this invention, the specified respective amounts oflead chloride and tin chloride are further combined with the presence ofsuitable amounts of one or more other metal (e.g. transition metal orrare earth metal) chlorides such as, but not limited to, nickelchloride, cobalt chloride, manganese chloride, cerium chloride andlanthanum chloride. For instance, the presence of up to 1 wt. % (even upto 1.5 wt. %) nickel chloride is not detrimental in terms of quality ofthe coating obtained after hot single-dip galvanization. Other metalchlorides that may be present include antimony chloride. Preferably theflux composition does not include a bismuth compound.

In another embodiment of this invention, the specified respectiverespective amounts of lead chloride and tin chloride are furthercombined with the presence of other additives, preferably functionaladditives participating in tuning or improving some desirable propertiesof the flux composition for performing the fluxing step in thecontinuous single dip galvanization process of the invention. Suchadditives are presented below.

For instance the flux composition used in the fluxing step of thisinvention may further comprise at least one non-ionic surfactant orwetting agent which, when combined with the other ingredients, iscapable of achieving a predetermined desirable surface tension.Essentially any type of nonionic surfactant, but preferably liquidwater-soluble, can be used. Examples thereof include ethoxylatedalcohols such as nonyl phenol ethoxylate, alkyl phenols such as TritonX-102 and Triton N101 (e.g. from Union Carbide), block copolymers ofethylene oxide and propylene oxide such as L-44 (from BASF), andtertiary amine ethoxylates derived from coconut, soybean, oleic ortallow oils (e.g. Ethomeen from AKZO NOBEL), polyethoxylated andpolypropoxylated derivatives of alkylphenols, fatty alcohols, fattyacids, aliphatic amines or amides containing at least 12 carbon atoms inthe molecule, alkylarene-sulfonates and dialkylsulfosuccinates, such aspolyglycol ether derivatives of aliphatic and cycloaliphatic alcohols,saturated and unsaturated fatty acids and alkylphenols, said derivativespreferably containing 3-10 glycol ether groups and 8-20 carbon atoms inthe (aliphatic) hydrocarbon moiety and 6-18 carbon atoms in the alkylmoiety of the alkylphenol, water-soluble adducts of polyethylene oxidewith poylypropylene glycol, ethylene-diaminopolypropylene glycolcontaining 1-10 carbon atoms in the alkyl chain, which adducts contain20-250 ethyleneglycol ether groups and/or 10-100 propyleneglycol ethergroups, and mixtures thereof. Such compounds usually contain from 1-5ethyleneglycol (EO) units per propyleneglycol unit. Representativeexamples are nonylphenol-polyethoxyethanol, castor oil polyglycolicethers, polypropylene-polyethylene oxide adducts,tributyl-phenoxypolyethoxy-ethanol, polyethylene-glycol andoctylphenoxypolyethoxyethanol. Fatty acid esters of polyethylenesorbitan (such as polyoxyethylene sorbitan trioleate), glycerol,sorbitan, sucrose and pentaerythritol, and mixtures thereof, are alsosuitable non-ionic surfactants. Low foaming wetting agents such as theternary mixtures described in U.S. Pat. No. 7,560,494 are also suitable.Commercially available non-ionic surfactants of the above-mentionedtypes include those marketed by Zschimmer & Schwarz GmbH & Co KG(Lahnstein, Germany) under the trade names OXETAL, ZUSOLAT and PROPETAL,and those marketed by Alfa Kimya (Istanbul, Turkey) under the trade nameNETZER SB II. Various grades of suitable non-ionic surfactants areavailable under the trade name MERPOL.

The hydrophilic-lipophilic balance (HLB) of said at least one nonionicsurfactant is not a critical parameter of this invention and may beselected by the skilled person within a wide range from 3 to 18, forinstance from 6 to 16. E.g. the HLB of MERPOL-A is 6 to 7, the HLB ofMERPOL-SE is 11, and the HLB of MERPOL-HCS is 15. Another feature of thenonionic surfactant is its cloud point (i.e. the temperature of phaseseparation as may me determined e.g. by ASTM D2024-09 standard testmethod; this behavior is characteristic of non-ionic surfactantscontaining polyoxyethylene chains, which exhibit reverse solubilityversus temperature in water and therefore “cloud out” at some point asthe temperature is raised; glycols demonstrating this behavior are knownas “cloud-point glycols”) which should preferably be higher than theflux working temperature as defined below with respect to the use of afluxing bath in a hot dip galvanization process. Preferably the cloudpoint of the nonionic surfactant should be higher than 90° C.

Suitable amounts of non-ionic surfactants are well known from theskilled person and usually range from 0.02 to 2.0 wt. %, preferably from0.5 to 1.0 wt. %, of the flux composition, depending upon the selectedtype of compound.

The flux compositions of the invention used in the fluxing step mayfurther comprise at least one corrosion inhibitor, i.e. a compoundinhibiting the oxidation of steel particularly in oxidative or acidicconditions. In one embodiment, the corrosion inhibitor includes at leastan amino group. Inclusion of such amino derivative corrosion inhibitorsin the flux compositions can significantly reduce the rate of ironaccumulation in the flux tank. By “amino derivative corrosion inhibitor”is meant herein a compound which inhibits the oxidation of steel andcontains an amino group. Aliphatic alkyl amines and quaternary ammoniumsalts (preferably containing 4 independently selected alkyl groups with1-12 carbon atoms) such as alkyl dimethyl quaternary ammonium nitrateare suitable examples of this type of amino compounds. Other suitableexamples include hexamethylenediamines. In another embodiment, thecorrosion inhibitor includes at least one hydroxyl group, or both ahydroxyl group and an amino group and are well known to those skilled inthe art. Suitable amounts of the corrosion inhibitor are well known fromthe skilled person and usually range from 0.02 to 2.0 wt. %, preferably0.1-1.5 wt. %, or 0.2-1.0 wt. %, depending upon the selected type ofcompound. The flux compositions of the invention may comprise both atleast one corrosion inhibitor and a nonionic surfactant or wetting agentas defined hereinabove.

In anyone of the above embodiments, the flux compositions of theinvention are preferably free from volatile organics, e.g. acetic acid,boric acid and methanol, especially those banned from galvanizationunits by legislation (safety, toxicity).

The flux compositions used in the fluxing step of the present inventionmay be produced by a wide range of methods. They can simply be producedby mixing, preferably thoroughly (e.g. under high shear), the essentialcomponents (i.e. zinc chloride, ammonium chloride, alkali or alkalineearth metal halides, lead chloride, tin chloride) and, if need be, theoptional ingredients (i.e. alkyl quaternary ammonium salt(s), othertransition or rare earth metal chlorides, corrosion inhibitor(s) and/ornonionic surfactant(s)) in any possible order in one or more mixingsteps. When lead chloride is present, the flux compositions used in thefluxing step of the invention may also be produced by a sequence of atleast two steps, wherein one step comprises the dissolution of leadchloride in ammonium chloride or sodium chloride or a mixture thereof,and wherein in a further step the solution of lead chloride in ammoniumchloride or sodium chloride or a mixture thereof is then mixed with theother essential components (i.e. zinc chloride, potassium chloride) and,if need be, the optional ingredients (as listed above) of thecomposition. In one embodiment of the latter method, dissolution of leadchloride is carried out in the presence of water. In another embodimentof the latter method, it is useful to dissolve an amount ranging from 8to 35 g/l lead chloride in an aqueous mixture comprising from 150 to 450g/l ammonium chloride and/or or sodium chloride and the balance beingwater. In particular the latter dissolution step may be performed at atemperature ranging from 55° C. to 75° C. for a period of time rangingfrom 4 to 30 minutes and preferably with stirring.

For use in the fluxing step of the process of this invention, a fluxcomposition according to any one of the above embodiments is preferablydissolved in water or an aqueous medium. Methods of dissolving in watera flux composition based on zinc chloride, ammonium chloride, alkalimetal chlorides and one or more chlorides of a transition metal are wellknown in the art. The total concentration of components of the fluxcomposition in the fluxing bath may range within very wide limits suchas 200-750 g/l, preferably 350-750 g/l, most preferably 500-750 g/l or600-750 g/l.

This fluxing bath, which is particularly adapted for the single dipcontinuous galvanizing process of the invention, should advantageouslybe maintained at a temperature within a range of 50° C.-90° C.,preferably 60° C.-90° C., most preferably 65° C.-85° C. throughout thefluxing step. The fluxing step is preferably performed for a period oftime (i.e. the average residence time of the steel long product in thefluxing bath) ranging from about 1 to 10 seconds. As is well known tothe skilled person, this period of time may widely vary from one longproduct to the other, depending upon operating parameters such as, butnot limited to, the composition of the fluxing bath, the composition ofthe metal (e.g. a low carbon or a high carbon steel, and the presenceand amount of metals other than iron), the shape and/or size of the longproduct, and the temperature of the fluxing bath. As a general rule,shorter times (e.g. from 1 to 6 seconds) are suitable for wires, whereaslonger times (closer to 10 seconds) are more suitable for instance forrods. Taking into account that the steel long product is usually movedalong the continuous production line, this kinetic parameter can also beexpressed in terms of a dipping speed from about 0.5 to 10 m/minute,preferably from 1 to 5 m/minute. Much higher speeds of 10-100 m/min,e.g. 20-60 m/min, can also be achieved.

Practically, any metal long product susceptible to corrosion, e.g. anytype of iron or steel long product may be treated in this way. Theshape, geometry or size of the metal long product are not criticalparameters of the present invention.

As well known in the art, it is important for the success of the wholegalvanization process that the surface of the steel long product besuitably cleaned before the fluxing step. Different techniques forachieving a desirable degree of surface cleanliness are well known inthe art. Different standards have been set forth regarding the desirabledegree of cleanliness, such as the maximal level of residual dirt of 0.6μg/cm² described in WO 03/057940. Conventional cleaning techniquesinclude alkaline cleaning, rinsing, pickling with a strong acid, anddrying, but are not limited thereto. For instance EP-A-2,281,912discloses cleaning the surface of a wire by passing it through a bathcontaining a phosphoric acid aqueous solution whereby said wire iscleaned by ultrasounds, followed by a vacuum drying stage. Although allthese procedures are well known, the following description is presentedfor the purpose of completeness.

Continuous alkaline cleaning can conveniently be carried out with anaqueous alkaline composition (e.g. a sodium or potassium hydroxideaqueous solution) also containing one or more phosphates (e.g. sodiumpoly-phosphate), carbonates (e.g. sodium carbonate) or silicates asbuilders as well as one or more various surfactant(s). The freealkalinity of such aqueous cleaners can vary broadly depending uponparameters such as, but not limited to, the type and concentration ofalkali hydroxide, the type and concentration of alkali salts. Theefficiency of the continuous alkaline cleaning step depends uponparameters such as, but not limited to, the temperature at which and theduration for which degreasing is carried out. According to a series ofexperiments, it has been found that the temperature during thecontinuous alkaline degreasing step may suitably range from about 40° C.to 65° C., for instance about 60° C. It has been found that the durationof the continuous alkaline degreasing step, i.e. the average period oftime wherein the steel long product passes through the degreasing bath,may suitably range from about 1 to 60 seconds, or up to 30 seconds, forinstance about 10 seconds, depending upon the degreasing temperature.Thus at an initial process step, the steel long product is submitted tocleaning (degreasing) in a degreasing bath. The latter mayadvantageously be assisted by an ultrasound generator provided in thealkali degreasing bath.

Then the steel long product is preferably rinsed. At a further step thesteel long product is submitted to a continuous pickling treatment andthen preferably rinsed. For instance the steel long product iscontinuously pickled by immersion into a bath of an aqueous stronglyacidic medium, e.g. a water-soluble inorganic acid such as hydrochloricacid, sulfuric acid, hydrofluoric acid, phosphoric acid, nitric acid andmixtures thereof in any suitable proportions. As is well known to theskilled person, the choice of the primary acid used for pickling dependsupon parameters such as the speed at which continuous pickling isdesired and the type of steel, in particular the alloy content in carbonsteel, from which the long product is made. The continuous pickling stepis usually performed at a temperature ranging from about 15° C. to 60″C, for instance 20° C., 25° C. or 40° C. Acid concentrations, e.g.hydrochloric acid concentrations, of about 5 wt. % to 20 wt. %, e.g. 12wt. % to 18 wt. %, are normally used, although more concentrated acidsare possible, depending upon the selected inorganic acid. The durationof the continuous pickling step, i.e. the average period of time whereinthe steel long product passes through the pickling bath, typicallyranges from about 3 to 30 seconds, more typically from 5 to 15 seconds,depending upon the acid and the temperature being used. Higher picklingtimes up to about 5 minutes may also be used.

In order to prevent over-pickling, it is also conventional to include inthe pickling liquid one or more corrosion inhibitor(s) such as definedherein-above, typically a cationic or amphoteric surface active agent.Typically, such one or more corrosion inhibitors may be present in thepickling bath in amounts ranging from 0.02 to 1.0 wt. %, for instance0.05-0.5 wt. %, depending upon the type of corrosion inhibitor. Thepickling bath may further include one or more halides, e.g. ferricchloride, ammonium fluoride and the like.

Pickling can be accomplished simply by dipping and moving the steel longproduct into a pickling tank containing the pickling bath. Additionalprocessing steps can also be used. For example, the steel long productcan be continuously or intermittently agitated either mechanically orultrasonically, and/or an electric current can be passed through it forelectro-pickling. The steel long product can also be submitted to ablasting step, for instance between alkaline degreasing and pickling,e.g. in a tumble blasting machine. These additional processing meansusually shorten the pickling time significantly. It is clear that thesepre-treatment steps may be repeated individually or by cycle if neededuntil the desirable degree of cleanliness is achieved.

Then shortly, preferably immediately, after the cleaning steps, themetal (e.g. steel) article is treated with, e.g. immersed into, afluxing bath comprising a fluxing composition with tin and leadchlorides amounts according to this invention in order to form anefficient and defect-free protective film on its surface as describedhereinbefore.

The fluxed steel long product, i.e. after immersion into the fluxingbath during the appropriate period of time and at the suitabletemperature, is preferably subsequently dried. Drying may be carried outby continuously passing the fluxed steel long product through a furnacehaving an air atmosphere, for instance a forced air stream, where it isheated at an air dryer temperature from about 220° C. to 300° C. untilthe long product surface exhibits a temperature ranging between 170° C.and 200° C., e.g. for a period of time ranging from about 1 to 3minutes. However It has also been surprisingly found that milder heatingconditions may be more appropriate when a fluxing composition, includingany particular embodiment thereof, is used in the fluxing step of thepresent invention. Thus it may be sufficient for the surface of thesteel long product to exhibit a temperature from 100° C. to 160° C., or120° C.-150° C. during the continuous drying step. This can be achievedfor instance by performing the drying step by using an induction heatingsystem or an infrared heating system, or a combination of both. In thisembodiment of the process, the heating temperature may range from 100°C. to 200° C., for instance from 110° C. to 160° C. This can also beachieved by using a poorly oxidative atmosphere during the continuousdrying step. In another embodiment, depending upon the selected dryingtemperature, drying may be continuously effected for a period of timeranging from about 3 to 10 minutes. In another embodiment, continuousdrying may be effected in specific gas atmospheres such as, but notlimited to a water-depleted air atmosphere, a water-depleted nitrogenatmosphere, or a water-depleted nitrogen-enriched air atmosphere (e.g.wherein the nitrogen content is above 20%).

At the next step of the continuous galvanization process, the fluxed anddried steel long product is submitted to a single dipping step into amolten aluminum-rich zinc-based galvanizing bath to form a protectivecoating thereon. As is well known, the dipping time of this singledipping step may be suitably defined depending upon a set of parametersincluding, but not restricted to, the size and shape of the article, thedesired coating thickness, the type of steel (low carbon or high carboncontent) and the exact composition of the zinc-based galvanization bath,in particular its aluminum content (when a Zn—Al alloy is used as thegalvanizing bath) or magnesium content (when a Zn—Al—Mg alloy is used asthe galvanizing bath). In an embodiment, the molten aluminum-richzinc-based galvanizing bath may comprise (a) from 4 to 24 wt % (e.g. 5to 20 wt. %) aluminum, (b) from 0 to 6 wt. % (e.g. 1 to 4 wt. %)magnesium, and (c) the rest being essentially zinc. In anotherembodiment of the present invention, the molten aluminum-rich zinc-basedgalvanizing bath may comprise from 0.5 to 1% by weight magnesium. Inanother embodiment of the present invention, the molten aluminum-richzinc-based galvanizing bath may comprise tiny amounts (i.e. below 1.0weight %) or trace amounts (i.e. unavoidable impurities) of otherelements such as, but not limited to, silicium, tin, lead, titanium orvanadium. In another embodiment of the present invention, the moltenaluminum-rich zinc-based galvanizing bath may be continuously orintermittently agitated during this treatment step. During this processstep, the zinc-based galvanizing bath is preferably maintained at atemperature ranging from 360° C. to 600° C. It has been surprisinglyfound that with a flux composition used in the fluxing step of theprocess of the present invention it is possible to lower the temperatureof the dipping step whilst obtaining thin protective coating layers of agood quality, i.e. defect-free and deemed to be capable of maintainingtheir protective effect for an extended period of time such as fiveyears or more, or even 10 years or more, depending upon the type ofenvironmental conditions (air humidity, temperature, pH, salinity, etc).Thus in one embodiment, the molten aluminum-rich zinc-based galvanizingbath is kept at a temperature ranging from 350° C. to 550° C., e.g. 380°C.-520° C. or 420° C.-530° C., the optimum temperature depending uponthe content of aluminum (and optionally magnesium) present in thealuminum-rich zinc-based galvanization bath.

In one embodiment, the thickness of the protective coating layerobtained by carrying out the continuous single dipping step on the steellong product of this invention may range from about 5 to 50 μm, forinstance from 8 to 30 μm. This can be appropriately selected by theskilled person, depending upon a set of parameters including thethickness and/or shape of the steel long product, the stress andenvironmental conditions which it is supposed to withstand, the expecteddurability in time of the protective coating layer formed, etc. Forinstance a 5-15 μm thick coating layer is suitable for a steel longproduct being less than 1.5 mm thick, and a 20-35 μm thick coating layeris suitable for a steel long product being more than 6 mm thick.

Finally, the steel long product may be removed from the galvanizing bathand cooled down. This cooling step may conveniently be carried outeither by dipping the galvanized metal article in water or simply byallowing it to cool down in air. Influence of the cooling kinetics iswell known in the art.

The present single dip galvanization process has been found to allowcontinuous deposition of thinner, more uniform, smoother and void-freeprotective coating layers on steel long products, especially when azinc-aluminum or zinc-aluminum-magnesium galvanizing bath with not morethan 95% zinc was used. Regarding roughness, the coating surface qualityis equal to or better than that achieved with a conventional HDG zinclayer according to EN ISO 1461 (i.e. with not more than 2% other metalsin the zinc bath). Regarding corrosion resistance, the coating layers ofthis invention achieve about 1,000 hours in the salt spray test of ISO9227 which is much better than the about 600 hours achieved with aconventional HDG zinc layer according to EN ISO 1461. This solves thetechnical problems outlined in the background of the present invention.

Moreover the process of the present invention is well adapted togalvanize steel long products made from a large variety of steel grades,in particular, but not limited to, steel long products having a carboncontent up to 0.25 weight %, a phosphorous content between 0.005 and 0.1weight % and a silicon content between 0.0005 and 0.5 weight %, as wellas stainless steels. The classification of steel grades is well known tothe skilled person, in particular through the Society of AutomotiveEngineers (SAE). In one embodiment of the present invention, the metalmay be a chromium/nickel or chromium/nickel/molybdenum steel susceptibleto corrosion. Suitable examples thereof are the steel grades known asAISI 304 (*1.4301), AISI 304L (1.4307, 1.4306), AISI 316 (1.4401), AISI316L (1.4404, 1.4435), AISI316Ti (1.4571), or AISI 904L (1.4539)[*1.xxxx=according to DIN 10027-2]. In another embodiment of the presentinvention, the metal may be a steel grade referenced as S235JR(according EN 10025) or S460MC (according EN 10149-2) or a carbon steelgrade known as 20MnB4 (1.5525).

The following examples are given for understanding and illustrating theinvention and should not be construed as limiting the scope of theinvention, which is defined only by the appended claims.

EXAMPLE 1

A 3 mm diameter wire made from a steel grade containing (by weight)0.06% carbon, 0.03% sulfur, 0.6% manganese, 0.15% silicium, 0.02%phosphorus, 0.1% chromium, 0.25% copper was processed as follows.

First, alkaline degreasing was continuously performed for 10 seconds ina degreasing bath comprising 50 g/l of a salt mix marketed under thetrade name Solvopol SOP by Lutter Galvanotechnik GmbH, and 1% by volumeof a tenside blend marketed under the trade name Emulgator SEP by LutterGalvanotechnik GmbH.

After rinsing the degreased wire was continuously passed through apickling bath containing 120 g/l hydrochloric acid, 10 ml/l of acorrosion inhibitor PM from Lutter Galvanotechnik GmbH, and 10 ml/l of atenside blend marketed under the trade name Emulgator DX by LutterGalvanotechnik GmbH. This pickling step was carried out at 40° C. for 10seconds.

After rinsing the degreased and pickled wire was continuously passedthrough a fluxing aqueous bath containing 550 g/l of a fluxingcomposition comprising (by weight) 60% zinc chloride, 20% ammoniumchloride, 3% sodium chloride, 12% potassium chloride, 4% tin chlorideand 1% lead chloride. This fluxing step was carried out at 72° C. for 6seconds.

The fluxed wire was then dried until its surface reaches 120° C. Finallygalvanization was performed with a zinc alloy containing 5% by weightaluminum and 1% by weight Mg. This galvanization step was carried out at420° C. for 6 seconds.

Quality of the resulting protective coating layer was assessed visuallyby a panel of three persons evaluating the percentage of the wiresurface that is perfectly coated by the aluminum-rich zinc alloy, i.e.free from defects such as pinholes and the like. The average note was98%.

EXAMPLE 2 General Procedure For Galvanization of a Steel Rod Grade HSA-F(C35)

A steel rod (thickness 8.0 mm) from a steel grade HSA-F (C35)(specifications by weight: 0.35-0.42% carbon, 0.15-0.35% silicium,0.6-0.9% manganese, max. 0.03% phosphorus, max. 0.04% sulfur) is treatedaccording the following procedure:

-   -   alkaline degreasing at 60° C. by means of SOLVOPOL SOP (50 g/l)        and a tenside mixture Emulgator Staal (10 g/l), both        commercially available from Lutter Galvanotechnik GmbH, for 30        minutes;    -   rinsing with water;    -   blasting in a tumble blasting machine during 30 minutes with an        angular steel grit (type GL80) with a projection speed of 65        m/s;    -   pickling in a hydrochloric acid based bath (composition: 18 wt.        % HCl, 10 ml/l of inhibitor PM and 10 ml/l Emulgator C75 both        available from Lutter Galvanotechnik GmbH) at 25° C. for 5        minutes;    -   rinsing with water;    -   fluxing the steel rod at 80° C. in a flux composition        (comprising 60 wt. % zinc chloride, 20 wt. % ammonium chloride,        3 wt. % sodium chloride, 12 wt. % potassium chloride, 4 wt % tin        chloride and 1 wt. % lead chloride) with a total salt        concentration of 650 g/l and in the presence of 2 ml/l Netzer 4        (a wetting agent from Lutter Galvanotechnik GmbH), by using an        extraction speed of 4 m/min;    -   drying until the steel rod surface temperature reaches 120° C.;    -   galvanizing the fluxed steel rod for 5 minutes at 530° C. with a        dipping speed of 4 m/min in a zinc based bath comprising 20.0        wt. % aluminum, 4.0 wt. % magnesium, 0.2% silicium and trace        amounts of lead, the balance being zinc; and    -   cooling down the galvanized steel plate in air.

This procedure has been found to provide a superior coating qualitysimilar to example 1. The following variants of this procedure alsoprovide superior coating quality:

-   -   -   Idem but with 5 minutes blasting, with 8 minutes fluxing,            and with galvanizing zinc bath at 510° C. during 5 or 10            minutes;        -   Idem but with 5 minutes blasting, with 8 minutes fluxing,            and with galvanizing zinc bath at 530° C. during 5, 10 or 15            minutes.

EXAMPLE 3 General Procedure For Galvanization of a Steel Rod Grade20MnB4

A steel rod (thickness 12.4 mm) from a steel grade 20MnB4 (with thefollowing contents by weight: 0.228% carbon, 0.197% silicium, 0.942%manganese, 0.011% phosphorus, 0.005% sulfur, 0.245% chromium, 0.036%nickel, 0.007% molybdenum, 0.038% aluminum and 0.057% copper) is treatedaccording the following procedure:

-   -   first alkaline degreasing at 60° C. by means of SOLVOPOL SOP (50        g/l) and a tenside mixture Emulgator Staal (10 g/l), both        available from Lutter Galvanotechnik GmbH, for 60 minutes;    -   rinsing with water;    -   pickling in a hydrochloric acid based bath (composition: 18 wt %        HCl, 10 g/l of fluorides from the salt NH4F.HF, 10 ml/l of        inhibitor PM and 10 ml/l Emulgator C75 from Lutter        Galvanotechnik GmbH) at 40° C. for 1 minute;    -   rinsing with water;    -   second alkaline degreasing at 60° C. for 5 minutes in a        degreasing bath with the same chemical composition as in the        first step;    -   rinsing with water;    -   cleaning in a solution with 100 g/l of Novaclean N and 2 ml/l of        Rodine A31 (a liquid anti-corrosive additive for acids available        from MAVOM, Schelle, Belgium), 10 ml/l of Netzer DX from Lutter        Galvanotechnik GmbH, at room temperature for 1 minute;    -   fluxing the steel rod at 80° C. for 10 minutes in a flux        composition comprising 60 wt. % zinc chloride, 20 wt. % ammonium        chloride, 3 wt. % sodium chloride, 12 wt. % potassium chloride,        4 wt. % tin chloride and 1 wt. % lead chloride) with a total        salt concentration of 650 g/l and in the presence of 2 ml/l        Netzer 4 (a wetting agent from Lutter Galvanotechnik GmbH), by        using an extraction speed of 4 m/min;    -   drying until the steel rod surface temperature reaches 120° C.;    -   galvanizing the fluxed steel rod for 10 minutes at 530° C. with        a dipping speed of 4 m/min in a zinc based bath comprising 20.0        wt. % aluminum, 4.0 wt. % magnesium, 0.2 wt. % silicium and        trace amounts of lead, the balance being zinc; and    -   cooling down the galvanized steel plate in air.

This procedure has been found to provide a superior coating qualitysimilar to example 1.

1. A continuous galvanization process for a steel long productcomprising one single dipping step consisting of dipping the said steellong product into a molten galvanizing bath comprising (a) from 4 to 24wt. % aluminum, (b) from 0 to 6 wt. % magnesium, and (c) the rest beingessentially zinc, wherein prior to said single dipping step the saidsteel long product has been submitted to one or more treatment stepsselected from the group consisting of alkaline cleaning, rinsing,pickling and drying, and furthermore to a fluxing step consisting ofdipping into a flux composition comprising (a) more than 40 and lessthan 70 wt. % zinc chloride, (b) 10 to 30 wt. % ammonium chloride, (c)more than 6 and less than 30 wt. % of a set of at least two alkali oralkaline earth metal halides, (d) from 0.1 to 2 wt. % lead chloride, and(e) from 2 to 15 wt. % tin chloride, provided that the combined amountsof lead chloride and tin chloride represent at least 2.5 wt. % of saidcomposition.
 2. A continuous galvanization process according to claim 1,wherein said flux composition further comprises at least one non-ionicsurfactant.
 3. A continuous galvanization process according to claim 1,wherein said flux composition further comprises at least one corrosioninhibitor.
 4. A continuous galvanization process according to claim 3,wherein said at least one corrosion inhibitor comprises at least onehydroxyl or amino group.
 5. A continuous galvanization process accordingto claim 1, wherein said set of at least two alkali or alkaline earthmetal halides is a set of at least two alkali metal chlorides includingsodium chloride and potassium chloride wherein the KCl/NaCl weight ratioranges from 0.2 to 1.0.
 6. A continuous galvanization process accordingto claim 1, wherein said set of at least two alkali or alkaline earthmetal halides is a set of at least two alkali metal chlorides includingsodium chloride and potassium chloride wherein the KCl/NaCl weight ratioranges from 1.0 to 8.0.
 7. A continuous galvanization process accordingto claim 1, wherein said flux composition further comprises at least onemetal chloride selected from the group consisting of nickel chloride,cobalt chloride, manganese chloride, cerium chloride, antimony chlorideand lanthanum chloride.
 8. A continuous galvanization process accordingto claim 1, wherein said flux composition comprises up to 1.5 wt. %nickel chloride.
 9. A continuous galvanization process according toclaim 1, wherein said flux composition is dissolved in water.
 10. Acontinuous galvanization process according to claim 9, wherein the totalconcentration of components of the flux composition in water ranges from200 to 750 g/l.
 11. A continuous galvanization process according toclaim 1, wherein said fluxing step is performed for a period of timeranging from 1 to 10 seconds.
 12. A continuous galvanization processaccording to claim 1, wherein said fluxing step is performed at atemperature ranging from 70° C. to 90° C.
 13. A continuous galvanizationprocess according to claim 1, wherein said alkaline cleaning step isperformed at a temperature ranging from 40° C. to 65° C. for a period oftime ranging from 1 second to 30 minutes.
 14. A continuous galvanizationprocess according to claim 1, wherein said pickling step is performed ata temperature ranging from 15° C. to 60° C. for a period of time rangingfrom 3 seconds to 5 minutes.
 15. A continuous galvanization processaccording to claim 1, wherein sais drying step is performed by means ofan induction heating system or an infrared heating system, or acombination of both, until the long product surface reaches 100° C. to150° C.
 16. A continuous galvanization process according to claim 1,wherein said steel long product is selected from the group consisting ofwires, rods, rails, structural shapes, bars and tubes.
 17. A galvanizedsteel long product obtained from a process according to claim 1, havinga protective coating layer with a thickness ranging from 5 to 50 μm. 18.A galvanized steel long product according to claim 17, being less than1.5 mm thick, wherein the protective coating layer has a thicknessranging from 5 to 15 μm.
 19. A galvanized steel long product accordingto claim 17, wherein the coating layer has a corrosion resistance of1000 hours in the salt spray test of ISO 9227,